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NASA Using Moonlight to Improve Satellite Accuracy – Boosted by That?


From NASA

NASA’s Lunar In Space Spectroscopy, or atmosphere-LUSI, flew aboard NASA’s ER-2 rover from March 12 to 16 to precisely measure the amount of light reflected off the Moon. Reflected moonlight is a steady source of light that researchers are leveraging to improve the accuracy and consistency of measurements between Earth-observing satellites.

“The Moon is extremely stable and is not to any great extent affected by Earth-based factors such as climate. Air-LUSI’s principal investigator, Kevin Turpie, a research professor at the University of Maryland, College Park, said.

The air-LUSI flights are part of NASA’s comprehensive satellite validation and calibration effort. The results will compliment ground-based sites, such as Playa . Railway Valley in Nevada, and together will provide the orbiting satellites with a powerful set of calibration data.

NASA has more than 20 Earth-observing satellites that provide researchers with a global view of the interconnected Earth system. Many of them measure light waves reflected, scattered, absorbed, or emitted by Earth’s surface, water, and atmosphere. This light includes visible light that humans see, as well as invisible ultraviolet and infrared wavelengths and everything in between. Like orchestral instruments, individual satellite instruments need to be “in tune” with each other for researchers to get the most out of their data. By using the Moon as a tuning fork, scientists can easily compare data from different satellites to look at global changes over long periods of time.

This cartoon about the electromagnetic spectrum shows how energy travels in waves;  Humans can only see visible light, but the entire spectrum is used by NASA instruments to observe Earth and beyond.
This electromagnetic spectrum shows how energy travels in waves; Humans can only see visible light, but the entire spectrum is used by NASA instruments to observe Earth and beyond. Credit: NASA

That’s where air-LUSI comes in. Developed in partnership with Institute of Standards and Technology (NIST), US Geological Survey and McMaster University, air-LUSI is a telescope that measures the amount of light reflected off the lunar surface to evaluate the amount of energy received by Earth-observing satellites from the moon’s light. It was mounted on an ER-2 aircraft managed by NASA’s Armstrong Flight Research Center and flown out of NASA’s Armstrong Flight Research Center in Palmdale, California. The ER-2 is an aircraft flying at 70,000 feet, above 95% of the atmosphere, that can scatter or absorb reflected sunlight. This allows air-LUSI to collect very precisely, NIST can track measurements are similar to those a satellite would take from orbit. To improve the accuracy of lunar reflectance models, atmospheric LUSI measurements are accurate with less than 1% measurement uncertainty. During flights in March, air-LUSI measured the Moon for four nights just before the Full Moon.

This aerial approach has the advantage of studying moonlight during different phases of the Moon and being able to bring the instrument back between flights for evaluation, maintenance, and repair if needed.

The cylindrical LUSI-Air Telescope was placed to measure the simulated Moon on the far side of the laboratory for testing and calibration before and after the flight campaign.
Shown is the air-LUSI telescope placed to measure the simulated Moon in the lab for testing and calibration before and after the flight campaign. Credit: Kevin Turpie

Improvements for better accuracy

The air-LUSI spectrometer is sealed in an enclosure that keeps the instrument at sea level temperature and pressure. Light captured by the telescope enters an integrator sphere that directs the light to a spectrometer, which is an instrument that measures the variation of light waves. Air-LUSI flies in similar flights for the first time in November 2019. Since then, the air-LUSI team has continued to improve the instrument’s accuracy.

The team improved the display inside so they could better test the device’s accuracy over a larger range of wavelengths, from ultraviolet to near infrared. They were also able to redesign the built-in sphere to eliminate the small effects of temperature changes.

“This will help the device take measurements with the level of accuracy more than 99% that we are looking for,” Turpie said.

Making these changes is a challenge. The delay of the COVID-19 pandemic prompted the chief engineer, who was working on updating and repairing equipment, to develop a new remote work plan. Both he and the principal investigator received special permission to deliver the parts directly to their homes so they could work on the equipment and prepare it for the 2022 flights.

Using the Moon as a common standard

Data from 2019 and 2022 together have the potential to assist scientists in making Earth observation satellite data in the ultraviolet to near-infrared range more consistent. In addition, a common Moon standard will make it easier to compare and refine current and future satellite observations. Plankton, Aerosol, Cloud, NASA’s Upcoming Ocean Ecosystem (SPEED) mission is planning to use the Moon as a common benchmark to make its observations more accurate and consistent with other satellite measurements of the Earth. Over the next decade, PACE and NASA’s future orbital sensors Earth System Observatory will help create a more cohesive picture of our planet.

“Having a common calibration source beyond Earth will help us achieve this goal,” says Turpie. “Once LUSI measurements in the atmosphere are used to improve the accuracy of the total amount of light coming from the Moon, we can make more precise measurements of the Earth using radio stations. observations in space now and in the future.”

Banner image: NASA ER-2 plane ready to refuel and prepare for flight. Image credit: Ken Ulrich

By Abby Graf
NASA Earth Science News Team



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